Re: [PATCH] af_unix: Allow Unix sockets to raise SIGURG

[Matthew Wilcox <willy@infradead.org>]

On Fri, Jan 29, 2021 at 09:56:48AM -0800, Shoaib Rao wrote:
> On 1/25/21 3:36 PM, Jakub Kicinski wrote:
> > On Fri, 22 Jan 2021 15:06:37 +0000 Matthew Wilcox (Oracle) wrote:
> > > From: Rao Shoaib <rao.shoaib@oracle.com>
> > > 
> > > TCP sockets allow SIGURG to be sent to the process holding the other
> > > end of the socket.  Extend Unix sockets to have the same ability.
> > > 
> > > The API is the same in that the sender uses sendmsg() with MSG_OOB to
> > > raise SIGURG.  Unix sockets behave in the same way as TCP sockets with
> > > SO_OOBINLINE set.
> > Noob question, if we only want to support the inline mode, why don't we
> > require SO_OOBINLINE to have been called on @other? Wouldn't that
> > provide more consistent behavior across address families?
> > 
> > With the current implementation the receiver will also not see MSG_OOB
> > set in msg->msg_flags, right?
> 
> SO_OOBINLINE does not control the delivery of signal, It controls how
> OOB Byte is delivered. It may not be obvious but this change does not
> deliver any Byte, just a signal. So, as long as sendmsg flag contains
> MSG_OOB, signal will be delivered just like it happens for TCP.

I don't think that's the question Jakub is asking.  As I understand it,
if you send a MSG_OOB on a TCP socket and the receiver calls recvmsg(),
it will see MSG_OOB set, even if SO_OOBINLINE is set.  That wouldn't
happen with Unix sockets.  I'm OK with that difference in behaviour,
because MSG_OOB on Unix sockets _is not_ for sending out of band data.
It's just for sending an urgent signal.

As you say, MSG_OOB does not require data to be sent for unix sockets
(unlike TCP which always requires at least one byte), but one can
choose to send data as part of a message which has MSG_OOB set.  It
won't be tagged in any special way.

To Jakub's other question, we could require SO_OOBINLINE to be set.
That'd provide another layer of insurance against applications being
surprised by a SIGURG they weren't expecting.  I don't know that it's
really worth it though.

One thing I wasn't clear about, and maybe you know, if we send a MSG_OOB,
does this patch cause this part of the tcp(7) manpage to be true for
unix sockets too?

       When out-of-band data is present, select(2) indicates the file descrip‐
       tor as having an exceptional condition and poll (2) indicates a POLLPRI
       event.